Discharge tube satisfactorily low in radio-interfering noise



DISCHARGE TUBE SATIFACTORILY LOW IN RADIO-'INTERFERING NC JISE HIDEO MIZUNO ET AL Filed April 17, 1967 v /500 FREQUENCY (KC) United States Patent 015cc 41/25,631 Us. Cl. 313-346 1 Claim Int. Cl. H01j 1/14, 19/06;H01k 1/04 ABSTRACT OF THE DISCLOSURE A discharge tube satisfactorily low in radio-interfering noise and equipped with cathodes coated with a cathode emitter consisting principally of oxides of barium, strontium and calcium and containing 0.05 to by Weight of a substance selected from the group consisting of cobalt boride and iron boride and further containing 1 to 8% by weight of a reducing metal having a high melting point and selected from the group consisting of zirconium, hafnium and tantalum, whereby a discharge tube low in noise intensity in the broadcasting frequency band is obtained.

The present invention relates to an improvement of discharge tubes, and more particularly to an improvement in low pressure mercury discharge tubes such as fluorescent lamps.

Oxide cathodes of the prior art utilized as electrodes in low pressure mercury vapor type discharge tubes such as fluorescent lamps were formed by coating double coils or so-called triple coils having a fine filament wound around said double coils with a compound carbonate consisting of barium carbonate, strontium carbonate and calcium carbonate, enclosing the coated structures in a discharge tube, thereafter subjecting these carbonates to thermal decomposition during the process of exhausting gas from said tube, thereby producing oxides of barium, strontium and calcium. The oxide layers thus produced were of high resistivity and poor thermal conductivity. Accordingly, a discharge tube incorporating cathodes thus obtained led, when lighted, to the development of cathode spots locally having a high temperature and such cathode spots constituted the centers of thermionic emission. When such discharge tube of the prior art was lighted, this temperature of the cathode spots did not make any substantial change due to their thermal inertia, either during the re-ignition or extinction of the arc of the cathodes in all AC cycles, but rather the cathode spots continued to have a high temperature. Therefore, at the time of re-ignition and extinction of the are at the cathodes of the prior art, the ther mionic current I naturally became larger than the discharge current I causing a negative potential to appear in front of the cathodes, and this led to the development of cathode oscillation such as re-ignition oscillation and extinction oscillation, resulting in the occurrence of a severe radio-interfering noise.

In order to avoid the generation of such interfering noise in fluorescent lamps, various attempts have been made to improve the thermal conductivity of the oxide layers of the electrodes used in such lamps. For example, Japanese patent publication No. 1,5 81/ 1964 proposes lowering the temperature of the cathode spots by restricting the thickness of the oxide layers to 30;; or less to substantially enhance the thermal conductivity of cathodes and 3,427,492 Patented Feb. 11, 1969 to thereby enlarge the cathode spots. However, this prior method is not desirable because it inevitably reduces the absolute volume of the electron-emitting oxides, and this, in turn, reduces the life of the discharge tube. Another attempt has been reported in Japanese patent publication No. 8,391/1965 which proposes enlargement of the distances between the pitches of coils to prevent an excess elevation of the temperature in coils due to mutual radiation occurring within the coils to thereby enlarge the cathode spots and lower the temperature thereof. Nevertheless, lamps having such arrangement produce radio-interfering noise in the order to 35 db when lighted and therefore, they still are unsatisfactory for use :as fluorescent lamps sufiiciently low in radio-interfering noise.

It is the object of the present invention to provide a fluorescent lamp which produces very little interference in radio receivers in the broadcasting frequency band of from 535 kc. to 1605 kc.

The inventors have discovered that a fluorescent lamp whose interference with radio receivers is restricted to 15 db or less and which, when used for practical purposes, is almost perfectly free from interfering noise and which is free from the development of blackening can be obtained by the use of a cathode emitter comprised of oxides of barium, strontium and calcium containing iron-cobalt borides having high melting points and having a markedly superior thermal conductivity as compared with that of an ordinary ionic crystal and also containing powder of a reducing metal selected from the group consisting of zirconium, hafnium, niobium and tantalum. Said powders of reducing metals, namely, zirconium, hafnium, niobium and tantalum satisfactorily prevent the metal borides from being oxidized during the thermal decomposition of the carbonates which is carried out during the process of exhausting gas from the discharge tubes.

According to the results of the experiment carried out by the inventors, a discharge tube according to the present invention which is equipped with oxide cathodes comprised of a mixture consisting of oxides of barium, strontium and calcium and containing, in a proportion of 1% by weight relative to said oxides, iron-cobalt borides corresponding to the composition formula of and further containing 3% by weight of zirconium relative to said oxides, showed a markedly superior effect of minimizing interfering noise. When a fluorescent lamp of the present invention was used while connecting a commonly known parallel condenser of 0.006 ,uf., the epochmaking ability that the noise intensity was reduced to 15 db or less throughout the frequency band of from 535 kc. to 1605 kc. was obtained as shown by curve 2 in the drawing. For comparisons sake, the result of the fluorescent lamp of the prior art is indicated by curve 1.

The drawing shows a comparison between the magnitudes of noise imparted to a radio-receiver by a fluorescent lamp embodying the present invention, curve 2, and by a conventional fluorescent lamp, curve 1.

The reasons the addition of iron cobalt borides to the oxide cathodes is able to reduce the cathode oscillation and lower the radio-interference, are not yet clear. In view of the finding in said experiment, however, that the cathode spots were larger and the temperature of the cathode spots was lower, than those of the conventional cathodes, it is conjectured that the thermal conductivity of the iron-cobalt borides which is higher than that of the oxides of barium, strontium and calcium may constitute one of the reasons. In view of the superior effect of minimizing the interfering noise afforded by the oxide cathodes employing iron-cobalt borides, it is considered that there may be also present a mutual relation of some sort or other between the iron-cobalt borides and the electron-emitting oxides.

The metal borides according to the present invention point to such iron-cobalt borides as those corresponding to the composition formula of {(1x)Fe-xCo} B These metal borides are mixed with carbonates of barium, strontium and calcium and this mixture is applied to the electrode coils. The resulting coated electrodes are then heated to 1200 C. during the process of exhausting gas from the fluorescent lamps. During this heating, the aforesaid carbonates are decomposed by the heat to form the so-called oxide cathodes. For this reason, it is necessary that the added metal borides have melting points of 1300 C. or higher and that they do not become oxidized when heated to the order of 1200 C. in a carbon dioxide gas atmosphere. Iron boride includes those having two different composition ratios, namely, Fe B and FeB. Cobalt boride includes those having two different composition ratios, namely, Co B and CoB. Their melting points are 1389 C., 1550 C., 1265 C. and 1350 C., respectively. Fe B and Co B have melting points which are somewhat lower than those of FeB and CoB. However, metal borides containing boron in a large proportion as in FeB and CoB reduce the life duration of the cathode emitter. For example, a fluorescent lamp employing an emitter containing 5% by weight of FeB has a life duration of about 2000 hours. In case 5% by weight of Fe B is added to the emitter, the life duration is in the order of 5000 hours. Where 1% by weight of Fe B is added, the life duration of the lamp is extended to 8000 hours or more. Therefore, the desirable iron-cobalt borides are represented by those having the composition formula of (Fe-Co) B. In case, however, the volume of boron added is smaller than that indicated by the chemical composition, this will lead to the development of free iron and cobalt and the cathodes become susceptible to oxidation during the process of decomposition as has been previously explained. It is, therefore, desirable to add boron in a proportion which is a little greater, i.e. y=1 to 1.15 in the aforesaid composition formula, than that required by the chemical composition. In case y 1.15, an ill effect on the duration of life of the cathodes becomes predominant and, therefore, the use of an excessive proportion of boron should be avoided. In case a simple substance of Fe B is used, this Fe B is oxidized to a certain extent during the thermal decomposition of the electrodes and as a result, an end-band type blackening is apt to occur after 1000 hours of lighting. This end-band type blackening takes place because the impure gases, especially oxygen, remaining in the tube are combined with mercury and deposited as mercury oxides in band form on the inner peripheral face of the lamp 3 cm. in front of the electrodes. While this end-band type blackening has nothing to do with the life of the fluorescent lamp, it markedly affects the value of the lamps as commercial goods.

Since Co B has a superior oxidation resistivity at high temperature to Fe B, there occurs hardly any blackening of this type in case C0 3 is added to the cathode emitter. In view of the fact, however, that Co B has a melting point of 1265" C., the use of Co B as a simple substance leads to the appearance of yellowish brown blackening in the terminal portions of the tube behind the electrodes after lighting for 1000 hours. In contrast to this, the fluorescent lamp using the composition formula {(1-x)Fe-xCo} B wherein 0.15 x0.6 and wherein to 60% of the iron of Fe B has been substituted with cobalt shows a great improvement in both the end-band and the blackening in the terminal portions of the tube as compared with the fluorescent lamp using Fe B or C0 B as a simple substance. In the aforesaid composition, when x 0.15, an end-band type blackening is apt to occur similarly as in the case of Fe B, while when x 0.6, the tube easily develops a blackening in the terminal portions as in the case of Co B.

In the oxide cathodes containing the borides of the present invention, the effect of minimizing the noise interference is noted when the cathodes contain 0.05% or more by weight of the borides. In order to avoid variance in the quality of goods during the process of their manufacture and to obtain goods of stable quality, it is preferred to add the borides in a proportion of the order of from 0.1 to 2%. While the effect of minimizing the noise interference can be equally obtained from the addition of borides of more than 2% by weight, the addition of borides in excess of 10% by weight means a reduced proportion of oxides of barium, strontium and calcium, and results in a reduced duration of life of the lamp, and therefore, the addition of borides in such excess proportion is not desirable. As has been described, the addition of an additive consisting of powder of one or more of the reducing metals having high melting points selected from the group consisting of zirconium, hafnium, niobium and tantalum which is to be contained in a proportion of the order of from 1 to 8% by weight in the oxides of the cathodes of the present invention is mandatory for the prevention of blackening of the lamp during lighting and also for the purpose of obtaining a prolonged duration of life of the lamp. The use of these additives in an amount less than 1% by weight leads to a failure in obtaining the desired effect, while the addition of an amount in excess of 8% by weight leads to a reduction in the ability of the cathodes to emit electrons therefrom, and therefore, the employment of these two extremes is not desirable.

Description will next be made to embodiments of the present invention.

EXAMPLE 1 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.8Fe-0.2Co) B 1 Zirconium 3 EXAMPLE 2 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 (0.5Fe-0.5Co) B 1 Zirconium 3 EXAMPLE 3 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 Iron-cobalt borides, (0.5Fe-0.5Co) B 1 Zirconium 2 Hafnium 1 EXAMPLE 4 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 Iron-cobalt borides (0.7Fe-0.3Co) B 1 Zirconium 2 Niobium 1 EXAMPLE 5 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 29 Iron-cobalt borides (O.5Fe'0.5Co) B 1 Zirconium 2 Tantalum 3 The mixtures shown in the above examples suspended in butyl acetate solutions of nitrocellulose, respectively, were applied to the double coils and to the triple coils of tungsten, and the resulting ooatal coils were subjected to thermal decomposition during the gas-exhausting process performed on the discharge tubes. Low pressure mercury vapor type discharge tubes having oxide cathodes consisting of these processed coils were thus fabricated. Ternary carbonates having the previously described composition ratios were used as the barium carbonate, strontium carbonate and calcium carbonate as are enumerated above.

We claim:

1. A discharge tube satisfactorily low in radio-interfering noise equipped with cathodes coated with a cathode emitter consisting of oxides consisting principally of oxides of barium, strontium and calcium, said oxides containing, in a proportion of from 0.05 to 10% by weight relative to said oxides, a substance having the composition of and consisting principally of iron-cobalt borides, the mixture of said oxides and said substance containing, in a proportion of from 1 to 8% by weight relative to said oxides, one or more reducing metals having high melting points and selected from the group consisting of zirconium, hafnium, niobium andtantalum.

Refcrences Cited JOHN W. HUCKERT, Primary Examiner.

J R. SHEWMAKER, Assistant Examiner.

US. 01. X.R. 

